The present invention relates to battery chargers, and more particularly to a battery charger incorporating an automatic fast/slow charge mode selector.
Various forms of battery chargers have been developed from time to time. Low voltage, e.g., one and one-half volts, three volts and so on, rechargable batteries are widely used in electrical and electronic devices, such as various forms of consumer electronic devices. Notable among these are electronic flash units, many of which use individual one and one half volt batteries or a suitable battery pack. It is desirable to be able to maintain such batteries in a charged condition, and it is particularly desirable on numerous occasions to be able to rapidly charge such batteries. Various forms of battery chargers are available on the market, many of which have only a fixed rate of charge. One presently available battery charger having both slow charge and fast charge capabilities is the Charge 15 manufactured by Vivitar Corporation. This form of charger incorporates suitable voltage and temperature sensing circuits to switch from the fast charge mode to the slow charge mode in the event the temperature of the battery, or the temperature in the vicinity of the battery, rises too high or the voltage of the battery rises too high. Either condition could result in damage to the battery.
In a fast automatic charger there usually is a thermal switch of some nature that terminates the fast charge before the battery temperature reaches a destructive value after the battery has been charged under high ambient temperature conditions. There likewise usually is a voltage cutoff system that terminates the fast charge before the voltage reaches a destructive value after recharge and at low ambient temperature conditions or when the battery was already fully charged. The use of both systems is known. See for example U.S. Pat. No. 4,006,397.
Prior fast/slow battery chargers have required manual switching devices whereby the user selects the fast charge mode or slow charge mode, or required various switching components for causing the charger to switch from the fast to the slow charge mode upon predetermined high voltage and high temperature conditions. Thus, some devices of this nature have been cumbersome to use and relatively expensive to manufacture.
According to the present invention, the charger of the present invention can use a thermal or other form of oscillator circuit for automatically switching between fast and slow charge modes so that the the mode desired by the user can be readily selected merely by inserting the battery into the charger at a time when the charger is in the desired mode, as contrasted to requiring the user to operate a switch or other device for selection of the charge mode. This provides for "time selection" of charge mode. Additionally, a fast/slow battery charger is provided wherein one thermal switch can be used to terminate the fast charge upon either or both of a high temperature condition or a high voltage condition.
A battery charger according to the present invention through the use of an oscillator system, has a number of features and advantages. When no battery is inserted into the charger, a thermal switch or other oscillatory type control system, will cycle at a slow rate (such as ten seconds per period) and alternately set the charger in the fast or slow mode to enable "time" selection by the user of the desired charge mode.
The time selection of charge mode obviates the necessity for a manual switch or reset button which the user normally would need to actuate to select the fast or slow charge mode. According to the present invention, before the battery to be recharged is inserted into the charger, the charger circuit alternately switches between the fast and slow charge modes, and the mode is indicated by a suitable indicator, such as a red light for fast charge and a green light for slow charge. After the battery is inserted, the charger stays in the mode it was in at the time the battery was inserted. This automatic time selection of mode not only obviates the need for a manually operable switch, but also obviates the problem occasioned by the user trying to hold the switch in a fast mode when the automatic detection circuitry (for temperature and/or voltage) indicates that the unit should be in the slow mode. With a charger constructed in accordance with the present invention, the user need only look at an indicator, and insert the battery when the indicator is in the fast or slow charge mode desired.
Furthermore, through the use of one thermal switch to terminate the charge upon either over temperature or over voltage conditions the usual additional electronic switch (such as an SCR, transistor, etc.) can be eliminated thereby reducing the cost and size of the charger. When the battery to be charged only has a few cells (such as two) the losses in the electronic control system or control switch can be as high as forty percent of the useful output of the charger, thereby creating unwanted heat and requiring a large transformer and larger power input. Furthermore, electronic elements which frequently are used in such chargers are unnecessarily fast and sensitive to spurious signals such as RF, and therefore in a simple consumer-oriented charger filter circuits and/or time delay or integration circuits are used thereby requiring additional components and the attendant complexity. A thermal switch according to the present invention is naturally slow and therefore is immune to power line surges, switch contact chattering, and other transients likely to occur in an uncontrolled environment of a consumer type of charger. The necessary latching capability required to hold the charger safely in a desired mode can be achieved by a single inexpensive component, such as a heating resistor, associated with a thermal switch. When the battery is removed from the charger, the latching component will automatically cool and the thermal switch will automatically be reset.
Accordingly, it is an object of the present invention to provide an improved battery charger.
Another object of the present invention is to provide a battery charger having an automatic time mode selector for automatically causing the charger to switch between fast and slow charge modes until a battery is inserted therein to be charged.
A further object of the present invention is to provide an improved form of battery charger using one thermal switch to terminate a fast charge mode upon the existence of either or both of a predetermined temperature condition and predetermined voltage condition.
These and other objects and features of the present invention will become better understood through a consideration of the following description taken in conjunction with the drawing in which the single FIGURE shows an exemplary embodiment of a circuit for a battery charger according to the present invention.